The present invention relates to a multi-layered unit including an electrode and a dielectric layer and, particularly, to a multi-layered unit suitable for fabricating a thin film capacitor having a small size, large capacitance and an excellent dielectric characteristic and suitable for fabricating an inorganic EL (electro-luminescence) device capable of emitting light having high luminescence.
Recently, the operating frequency of LSIs (Large Scale Integrated circuits), typically CPUs (Central Processing Units), has become higher and higher. In the LSI having a high operating frequency, power supply noise is very likely to be generated, and once power supply noise occurs, a voltage drop occurs due to parasitic resistance and parasitic inductance of the power supply wiring, causing the LSI to operate erroneously.
In order to prevent such a voltage drop caused by power supply noise, a decoupling capacitor is generally connected between the terminals of the LSI power supply. In the case where a decoupling capacitor is connected between the terminals of the LSI power supply, the impedance of the power supply wiring decreases to effectively prevent voltage drop caused by power supply noise.
The impedance required of the power supply wiring is proportional to the operating voltage of the LSI and inversely proportional to the integration density of the LSI, the switching current of the LSI and the operating frequency of the LSI. Therefore, in current LSIs, which have high integration density, low operating voltage and high operating frequency, the impedance required of the power supply wiring is extremely low.
In order to achieve such low impedance of the power supply wiring, it is necessary to increase the capacitance of the decoupling capacitor and considerably lower the inductance of the wiring connecting the terminals of the LSI power supply and the decoupling capacitor.
As a decoupling capacitor having a large capacitance, an electrolytic capacitor or a multilayer ceramic capacitor is generally employed. However, since the size of an electrolytic capacitor or multilayer ceramic capacitor is relatively large, it is difficult to integrate it with an LSI. Therefore, the electrolytic capacitor or multilayer ceramic capacitor has to be mounted on a circuit substrate independently of the LSI and, as a result, the length of wiring for connecting the terminals of the LSI power supply and the decoupling capacitor is inevitably long. Accordingly, in the case where an electrolytic capacitor or a multilayer ceramic capacitor is employed as a decoupling capacitor, it is difficult to lower the inductance of the wiring for connecting the terminals of the LSI power supply and the decoupling capacitor.
In order to shorten the wiring for connecting the terminals of the LSI power supply and the decoupling capacitor, use of a thin film capacitor having a smaller size than that of an electrolytic capacitor or a multilayer ceramic capacitor is suitable.
Japanese Patent Application Laid Open No. 2001-15382 discloses a thin film capacitor having a small size and large capacitance which employs PZT, PLZT, (Ba, Sr) TiO3 (BST), Ta2O5 or the like as a dielectric material.
However, the thin film capacitor employing any one of the above mentioned materials is disadvantageous in that the temperature characteristic thereof is poor. For example, since the dielectric constant of BST has a temperature dependency of xe2x88x921000 to xe2x88x924000 ppm/xc2x0 C., in the case where BST is employed as a dielectric material, the capacitance of the thin film capacitor at 8xc2x0 C. varies between xe2x88x926% and xe2x88x9224% in comparison with that at 20xc2x0 C. Therefore, a thin film capacitor employing BST as a dielectric material is not suitable for use as a decoupling capacitor for a high operating frequency LSI whose ambient temperature frequently reaches 80xc2x0 C. or higher owing to heat generated by electric power consumption.
Furthermore, the dielectric constant of a dielectric thin film formed of any one of the above mentioned materials decreases as the thickness thereof decreases and the capacitance thereof greatly decreases when an electric field of 100 kV/cm, for example, is applied thereto. Therefore, in the case where any one of the above mentioned materials is used as a dielectric material for a thin film capacitor, it is difficult to simultaneously make the thin film capacitor small and the capacitance thereof great.
In addition, Moreover, since the surface roughness of a dielectric thin film formed of any one of the above mentioned materials is high, its insulation performance tends to be lowered when formed thin.
It might be thought possible to overcome these problems by using a bismuth layer structured compound as a dielectric material for a thin film capacitor. The bismuth layer structured compound is discussed by Tadashi Takenaka in xe2x80x9cStudy on the particle orientation of bismuth layer structured ferroelectric ceramics and their application to piezoelectric or pyroelectric materialsxe2x80x9d Engineering Doctoral Thesis at the University of Kyoto (1984), Chapter 3, pages 23 to 36.
It is known that the bismuth layer structured compound has an anisotropic crystal structure and behaves as a ferroelectric material but that the bismuth layer structured compound exhibits only weak property as a ferroelectric material and behaves like as a paraelectric material along a certain axis of orientation.
The property of the bismuth layer structured compound as a ferroelectric material is undesirable when the bismuth layer structured compound is utilized as a dielectric material for a thin film capacitor since it causes variation in dielectric constant. Therefore, when a bismuth layer structured compound is used as a dielectric material for a thin film capacitor, it is preferable that its paraelectric property can be fully exhibited.
Therefore, a need has been felt for the development of a thin film capacitor of small size, large capacitance and excellent dielectric characteristic that has a dielectric layer in which a bismuth layer structured compound oriented in the axis of orientation along which the bismuth layer structured compound exhibits only weak property as a ferroelectric material and behaves like a paraelectric material.
On the other hand, it is necessary in order to fabricate an inorganic EL (electro-luminescence) device for emitting light having high luminescence to provide a dielectric layer having a high insulating property between an electrode and an inorganic EL device and it is therefore required to develop an inorganic EL device provided with a dielectric layer in which a bismuth layer structured compound oriented in the axis of orientation along which the bismuth layer structured compound exhibits only weak property as a ferroelectric material and behaves like a paraelectric material.
It is therefore an object of the present invention to provide a multi-layered unit suitable for fabricating a thin film capacitor having a small size, large capacitance and an excellent dielectric characteristic and suitable for fabricating an inorganic EL (electro-luminescence) device capable of emitting light having high luminescence.
The above and other objects of the present invention can be accomplished by a multi-layered unit constituted by forming on a support substrate formed of a material on which crystals cannot be epitaxially grown, a buffer layer, which is formed of a material having an anisotropic property and enabling epitaxial growth of crystals of a conductive material thereon to form an electrode layer and is oriented in the [001] direction, the electrode layer formed by epitaxially growing crystals of a conductive material and oriented in the [001] direction, and a dielectric layer formed by epitaxially growing a dielectric material containing a bismuth layer structured compound on the electrode layer and oriented in the [001] direction in this order.
In the present invention, the [001] direction as termed herein means the [001] direction of a cubic crystal, a tetragonal crystal, a monoclinic crystal or an orthorhombic crystal.
According to the present invention, since the electrode layer is formed by epitaxially growing crystals of a conductive material on the buffer layer, which is formed of a material having an anisotropic property and enabling epitaxial growth of crystals of a conductive material thereon to form an electrode layer and is oriented in the [001] direction, it is possible to reliably orient the electrode layer in the [001] direction.
Further, according to the present invention, since the dielectric layer of a dielectric material containing a bismuth layer structured compound is formed by epitaxially growing the dielectric material containing the bismuth layer structured compound on the electrode layer oriented in the [001] direction, it is possible to easily orient the dielectric layer in the [001] direction, thereby improving the c axis orientation characteristic.
Therefore, according to the present invention, since the c axis of the bismuth layer structured compound contained in the dielectric layer can be oriented so as to be perpendicular to the electrode layer, in the case of, for example, providing an upper electrode on the dielectric layer and applying a voltage between the electrode layer and the upper electrode, the direction of the electric field substantially coincides with the c axis of the bismuth layer structured compound contained in the dielectric layer. Accordingly, since the ferroelectric property of the bismuth layer structured compound contained in the dielectric layer can be suppressed and the paraelectric property thereof can be fully exhibited, it is possible to fabricate a thin film capacitor having a small size and large capacitance.
Further, according to the present invention, since the dielectric layer of the dielectric material containing the bismuth layer structured compound whose c axis orientation is improved has a high insulating property, it is possible to form the dielectric layer thinner and therefore make a thin film capacitor much smaller.
Furthermore, since the dielectric layer of the dielectric material containing the bismuth layer structured compound whose c axis orientation is improved has a high insulating property, it is possible to cause an inorganic EL device to emit light in a desired manner and fabricate an inorganic EL device capable of emitting light having high luminescence by disposing the inorganic EL device on the dielectric layer of the multi-layered unit according to the present invention, disposing another electrode on the inorganic EL device and applying a voltage between the electrode layer and another electrode.
In the present invention, the dielectric material containing the bismuth layer structured compound may contain unavoidable impurities.
In the present invention, it is sufficient for the support substrate to be formed of a material on which crystals cannot be epitaxially grown and the material for forming the support substrate is not particularly limited. An amorphous substrate made of fused quartz or the like, a polycrystal substrate made of ceramic or the like, a heat-resistant glass substrate, a resin substrate or the like can be used as the support substrate.
In the present invention, the multi-layered unit includes a buffer layer oriented in the [001] direction, namely, the c axis direction on the support substrate. The buffer layer serves to ensure easy formation of an electrode layer thereon so as to be oriented in the [001] direction, namely, the c axis direction,
In the case of directly forming an electrode layer made of platinum or the like on the support substrate made of fused quartz or the like, since the electrode layer tends to be oriented in the [111] direction, it is difficult to epitaxially grow a dielectric layer of a dielectric material containing a bismuth layer structured compound on the electrode layer to form a dielectric layer of the dielectric material containing the bismuth layer structured compound oriented in the [001] direction, namely, the c axis direction. However, in the present invention, since the electrode layer is formed on the buffer layer, which is formed of a material having an anisotropic property and enabling epitaxial growth of crystals of a conductive material to form an electrode layer and is oriented in the [001] direction, namely, the c axis direction, the electrode can be formed so as to be reliably oriented in the [001] direction, namely, the c axis direction.
In the present invention, the material for forming the buffer layer is not particularly limited insofar as it has an anisotropic property and enables epitaxial growth of crystals of a conductive material thereon to form an electrode layer, and a bismuth layer structured compound and a copper oxide superconductor having a CuO2 plane can be preferably used for forming the buffer layer.
The bismuth layer structured compound has a composition represented by the stoichiometric compositional formula: (Bi2O2)2+ (Amxe2x88x921BmO3m+1)2xe2x88x92 or Bi2Amxe2x88x921BmO3m+3, where the symbol m is a natural number, the symbol A is at least one element selected from a group consisting of sodium (Na), potassium (K), lead (Pb), barium (Ba), strontium (Sr), calcium (Ca) and bismuth (Bi), and the symbol B is at least one element selected from a group consisting of iron (Fe), cobalt (Co), chromium (Cr), gallium (Ga), titanium (Ti), niobium (Nb), tantalum (Ta), antimony (Sb), vanadium (V), molybdenum (Mo) and tungsten (W). In the case where the symbol A and/or B includes two or more elements, the ratio of the elements can be arbitrarily determined.
As shown in FIG. 1, the bismuth layer structured compound has a layered structure formed by alternately laminating perovskite layers 1 each including perovskite lattices 1a made of (mxe2x88x921) ABO3 and (Bi2O2)2+ layers 2.
The number of laminates each consisting of the perovskite layer 1 and the (Bi2O2)2+ layer 2 is not particularly limited and it is sufficient for the bismuth layer structured compound to include at least one pair of (Bi2O2)2+ layers 2 and one perovskite layer 1 sandwiched therebetween.
The c axis of the bismuth layer structured compound means the direction obtained by connecting the pair of (Bi2O2)2+ layers 2, namely, the [001] direction.
Among the bismuth layer structured compounds represented by the above stoichiometric compositional formula, a bismuth layer structured compound where the symbol m is equal to 3 in the general stoichiometric compositional formula thereof, namely, that represented by the stoichiometric compositional formula: (Bi2O2)2+ (A3B4O13)2xe2x88x92 or Bi2A3B4O15, is most preferably employed since it can be easily oriented in the [001] direction, namely, the c axis direction,
As a copper oxide superconductor having a CuO2 plane, compounds represented by the stoichiometric compositional formula: YBa2Cu3O7-xcex4 and Bi2Sr2CuO6 are preferably used for forming the buffer layer.
In the present invention, it is not absolutely necessary for the degree F of orientation in the [001] direction, namely, c axis orientation of the material having an anisotropic property and contained in the buffer layer to be 100% but it is sufficient for the degree F of c axis orientation of the material to be equal to or more than 80%. It is more preferable for the degree of c axis orientation of the material to be equal to or more than 90% and it is much more preferable for the degree of c axis orientation of the material to be equal to or more than 95%.
The degree F of the c axis orientation of the material having an anisotropic property is defined by the following formula (1).
F=(Pxe2x88x92P0)/(1xe2x88x92P0)xc3x97100xe2x80x83xe2x80x83(1)
In formula (1), P0 is defined as the X-ray diffraction intensity of polycrystal whose orientation is completely random in the c axis direction, namely, the ratio of the sum xcexa3I0 (001) of reflection intensities I0 (001) from the surface of [001] of polycrystal whose orientation is completely random to the sum xcexa3I0 (hk1) of reflection intensities I0 (hk1) from the respective crystal surfaces of [hk1] thereof (xcexa3I0 (001)/xcexa3I0 (hk1), and P is defined as X-ray diffraction intensity of a material having an anisotropic property in the c axis direction, namely, the ratio of the sum xcexa3I (001) of reflection intensities I (001) from the surface of [001] of the material having an anisotropic property to the sum xcexa3I (hk1) of reflection intensities I (hk1) from the respective crystal surfaces of [hk1] thereof (xcexa3I (001)/xcexa3I (hk1)). The symbols h, k and 1 can each assume an arbitrary integer value equal to or larger than 0.
In the above formula (1), since P0 is a known constant, when the sum xcexa3I (001) of reflection intensities I (001) from the surface of [001] of the material having an anisotropic property and the sum xcexa3I (hk1) of reflection intensities I (hk1) from the respective crystal surfaces of [hk1] are equal to each other, the degree F of the c axis orientation of the material having an anisotropic property is equal to 100%.
In the present invention, the buffer layer can be formed using any of various thin film forming processes such as a vacuum deposition process, a sputtering process, a pulsed laser deposition process (PLD), a metal organic chemical vapor deposition process (MOCVD), a chemical solution deposition process (CSD process) such as a metal-organic decomposition process (MOD) and a sol-gel process or the like. Particularly, in the case where the buffer layer has to be formed at a low temperature, a plasma CVD process, a photo-CVD process, a laser CVD process, a photo-CSD process, a laser CSD process or the like is preferably used for forming the buffer layer.
In the present invention, the multi-layered unit includes an electrode layer of a conductive material oriented in the [001] direction, namely, the c axis direction on the buffer layer.
In the present invention, since the electrode layer is formed by epitaxially growing crystals of a conductive material on the buffer layer, which is formed of a material having an anisotropic property and enabling epitaxial growth of crystals of a conductive material thereon to form an electrode layer and is oriented in the [001] direction, namely, the c axis direction, it is possible to reliably orient an electrode layer in the [001] direction, namely, the c axis direction.
In the present invention, the material for forming the electrode layer is not particularly limited and can be a metal such as platinum (Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), iridium (Ir), gold (Au), silver (Ag), copper (Cu), nickel (Ni) or the like, an alloy containing at least one of these metal as a principal component, a conductive oxide such as NdO, NbO, RhO2, OsO2, IrO2, RuO2, SrMoO3, SrRuO3, CaRuO3, SrVO3, SrCrO3, SrCoO3, LaNiO3, Nb doped SrTiO3 or the like or a mixture of these.
It is preferable to select from among these materials a material having a small lattice mismatch with the material having an anisotropic property and forming the buffer layer 4 and the bismuth layer structured compound for forming a dielectric layer.
In the present invention, the electrode layer can be formed using any of various thin film forming processes such as a vacuum deposition process, a sputtering process, a pulsed laser deposition process (PLD), a metal organic chemical vapor deposition process (MOCVD), a chemical solution deposition process (CSD process) such as a metal-organic decomposition process (MOD) and a sol-gel process or the like.
In the present invention, the multi-layered unit includes a dielectric layer of a dielectric material containing a bismuth layer structured compound oriented in the [001] direction, namely, the c axis direction on the electrode layer.
In the present invention, the dielectric layer is formed by epitaxially growing a dielectric material containing a bismuth layer structured compound on the electrode layer.
Since the dielectric layer is formed by epitaxially growing a dielectric material containing a bismuth layer structured compound on the electrode layer oriented in the [001] direction, it is possible to easily orient the bismuth layer structured compound contained in the dielectric layer in the [001] direction, namely, the c axis direction. Therefore, in the case where a thin film capacitor is fabricated using the multi-layered unit according to the present invention, since the bismuth layer structured compound contained in the dielectric layer does not function as a ferroelectric material but functions as a paraelectric material, it is possible to fabricate a thin film capacitor having a small size and large capacitance.
In the present invention, it is not absolutely necessary for the degree F of orientation in the [001] direction, namely, c axis orientation of the bismuth layer structured compound contained in the dielectric layer to be 100% and it is sufficient for the degree F of c axis orientation to be equal to or more than 80%. It is more preferable for the degree of c axis orientation of the bismuth layer structured compound to be equal to or more than 90% and it is much more preferable for the degree of c axis orientation of the bismuth layer structured compound to be equal to or more than 95%.
The degree F of the bismuth layer structured compound is defined by the formula (1).
The dielectric characteristic of a dielectric layer can be markedly improved by orienting the bismuth layer structured compound in the [001] direction, namely, the c axis direction in this manner.
More specifically, in the case where a thin film capacitor is fabricated by forming, for example, an upper electrode on the dielectric layer of the multi-layered unit according to the present invention, even if the thickness of the dielectric layer is equal to or thinner than, for example, 100 nm, a thin film capacitor having a relatively high dielectric constant and low loss (tan xcex4) can be obtained. Further, a thin film capacitor having an excellent leak characteristic, an improved breakdown voltage, an excellent temperature coefficient of the dielectric constant and an excellent surface smoothness can be obtained.
In the present invention, among bismuth layer structured compounds usable for forming the buffer layer, a bismuth layer structured compound having an excellent characteristic as a capacitor material can be used for forming the dielectric layer. The bismuth layer structured compound for forming the dielectric layer is not particularly limited insofar as it has an excellent characteristic as a capacitor material, and a bismuth layer structured compound where the symbol m is equal to 3 in the general stoichiometric compositional formula thereof namely, that represented by the stoichiometric compositional formula: (Bi2O2)2+ (A2B3O10)2xe2x88x92 or Bi2A2 B3O12, is preferably used since it has an excellent characteristic as a capacitor material.
In the present invention, it is particularly preferable that the bismuth layer structured compound contained in the dielectric layer has a composition represented by the stoichiometric compositional formula: CaxSr(1-x)Bi4Ti4O15, where x is equal to or larger than 0 and equal to or smaller than 1. If the bismuth layer structured compound having such a composition is used, a dielectric layer having a relatively large dielectric constant can be obtained and the temperature characteristic thereof can be further improved.
In the present invention, parts of the elements represented by the symbols A or B in the stoichiometric compositional formula of the bismuth layer structured compound contained in the dielectric layer are preferably replaced with at least one element Re (yttrium (Y) or a rare-earth element) selected from the group consisting of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).
The preferable amount of replacement by the element Re depends upon the value of the symbol m. For example, in the case where the symbol m is equal to 3, in the compositional formula: Bi2A(2-x)RexB3O12, x is preferably equal to or larger than 0.4 and equal to or smaller than 1.8 and more preferably equal to or larger than 1.0 and equal to or smaller than 1.4. If the amount of replacement by the element Re is determined within this range, the Curie temperature (phase transition temperature from ferroelectric to paraelectric) of the dielectric layer can be controlled preferably to be equal to or higher than xe2x88x92100xc2x0 C. and equal to or lower than 100xc2x0 C. and more preferably to be equal to or higher than xe2x88x9250xc2x0 C. and equal to or lower than 50xc2x0 C. If the Curie point is equal to or higher than xe2x88x92100xc2x0 C. and equal to or lower than 100xc2x0 C., the dielectric constant of the dielectric thin film 6 increases. The Curie temperature can be measured by DSC (differential scanning calorimetry) or the like. If the Curie point becomes lower than room temperature (25xc2x0 C.), tan xcex4 further decreases and, as a result, the loss value Q further increases.
Furthermore, in the case where the symbol m is equal to 4, in the compositional formula: Bi2A(3-x)RexB4O15, x is preferably equal to or larger than 0.01 and equal to or smaller than 2.0 and more preferably equal to or larger than 0.1 and equal to or smaller than 1.0.
Although the dielectric layer of the multi-layered unit according to the present invention has an excellent leak characteristic even if it does not contain the element Re, it is possible to further improve the leak characteristic by replacing part of the elements represented by the symbols A or B with the element Re.
For example, even in the case where no part of the elements represented by the symbols A or B in the stoichiometric compositional formula of the bismuth layer structured compound is replaced with element Re, the leak current measured at the electric filed strength of 50 kV/cm can be controlled preferably to be equal to or lower than 1xc3x9710xe2x88x927 A/cm2 and more preferably to be equal to or lower than 5xc3x9710xe2x88x928 A/cm2 and the short circuit ratio can be controlled preferably to be equal to or lower than 10% and more preferably to be equal to or lower than 5%. However, in the case where parts of the elements represented by the symbols A or B in the stoichiometric compositional formula of the bismuth layer structured compound are replaced with element Re, the leak current measured under the same condition can be controlled preferably to be equal to or lower than 5xc3x9710xe2x88x928 A/cm2 and more preferably to be equal to or lower than 1xc3x9710xe2x88x928 A/cm2 and the short circuit ratio can be controlled preferably to be equal to or lower than 5% and more preferably to be equal to or lower than 3%.
In the present invention, the dielectric layer can be formed using any of various thin film forming processes such as a vacuum deposition process, a sputtering process, a pulsed laser deposition process (PLD), a metal organic chemical vapor deposition process (MOCVD), a chemical solution deposition process (CSD process) such as a metal-organic decomposition process (MOD) and a sol-gel process or the like. Particularly, in the case where the dielectric layer has to be formed at a low temperature, a plasma CVD process, a photo-CVD process, a laser CVD process, a photo-CSD process, a laser CSD process or the like is preferably used for forming the dielectric layer.
The above and other objects and features of the present invention will become apparent from the following description made with reference to the accompanying drawings.